I own the land the cave is on, and refuse to let anyone fix the cave, unless you pay me. Blackmail?

Yes.

What about if the only reason the cave entrance was crumbling was because I diverted a river. I didn't do this with the intent of causing the entrance to crumble (I have other reasons), but I was fully aware of this side effect, and let it happen. Blackmail?

No. Assuming that protecting the cave is free (because someone else wants to come and do it) the diverting of the river is not itself blackmail. Subsequent blackmail is still blackmail. It becomes close to blackmail if the cost to repair the cave is greater than the value of the river diversion. Depending on the various value instantiations possible this example does a good job of exemplifying the 'blackmail vs gains to trade' blurry boundary.

What if I approached you before I diverted the river, and asked for money then. I still want to divert the river, but if you pay up enough, I will desist from it. Blackmail?

No, assuming the right values for various outcomes this is exactly what "split the gains from trade equally" looks like. The inconvenience for keeping the secret is divided evenly. Another excellent example because to most humans this will feel like extortion and if the numbers are tweaked a little it could be extortion by my standards too.